Field of the Disclosure
This disclosure pertains to multilayer films and authentication labels.
Description of the Related Art
Holographic images may be used for identification and authentication of products and may be applied directly to the products or the product packaging. The most common hologram used is a surface-relief hologram, formed by a patterning and stamping process in which a relief grating is formed on the surface of a recording medium, such as a photoresist material. In a surface-relief hologram, interference planes are predominantly perpendicular to the recording surface and the surface-relief grating of the recording medium provides for the diffraction of incident light, enabling the formation of the observed holographic image. As basic holographic technology becomes more widely available, however, such a simple hologram may be more easily counterfeited and applied to non-authentic products. Therefore, a surface-relief hologram has limited value as an authentication and/or security device.
Another type of hologram is a volume-phase hologram, in which a recording medium is a photosensitive material, such as a silver halide emulsion, a dichromated gelatin or a photopolymer. In a volume-phase hologram, the interference planes are not limited to just being perpendicular to the recording surface (as in surface relief holograms), but can be at a variety of angles with respect to the recording surface. Photo-imaging creates interference fringes throughout the depth of the recording medium that diffract light and create the observed holographic image. Unlike a surface-relief hologram, which does not reflect light, but only diffracts light in transmission, a volume-phase hologram can do both. Volume-phase holograms are more difficult and costly to produce than surface-relief holograms, but enable the production of a more detailed image that has greater three-dimensional character, complete with parallax, and is far more challenging to counterfeit.
Holographic recording mediums typically include at least one support film that supports a layer of the photosensitive material; and, particularly for photopolymeric holographic recording mediums, can include a coversheet or protective film adjacent a side of the photosensitive material that is opposite the support film. Since a layer of the photosensitive material for holographic recording is frangible and lacks sufficient mechanical strength, the support film and the optional coversheet provides sufficient integrity to keep the frangible photosensitive layer intact and allow the holographic recording medium to be handled throughout the process to convert the recording medium into a holographic image layer containing desired holographic image without damage to the layer or the image. In general, multilayer films that contain the holographic image layer that still is frangible also include a support and/or protective layer to prevent damage of the holographic image layer during machine operation for automated application to form a label and/or for attachment to label stock or a substrate. In general, an authentication label that includes the frangible holographic image layer also includes at least a protective film layer as a top or outermost layer that protects the underlying layers and particularly the holographic image layer. The protective film layer is affixed to and is part of the authentication label to protect the label from damage throughout its lifetime. A polyester film is often used to protect the holographic image layer and the authentication label from damage.
A volume-phase hologram is often produced as an authentication label in the form of a multilayer film that is subsequently applied to a product using an adhesive layer. In some cases, counterfeiters will attempt to remove an authentication label from a genuine product or product packaging and reapply it to a non-authentic good. Often this is done by using heat (e.g., a heat gun) to soften the adhesive layer, and then carefully scraping the label off. The removal of the label can be facilitated by the presence of a protective film and/or support film, which allows the holographic image layer to be lifted or separated from the product or package without damage.
It would be desirable to produce an authentication label that cannot be readily removed from one substrate and reapplied to another. It also would be desirable to produce an authentication label in which the holographic image layer remains intact and undamaged for application to a substrate, but the holographic image layer is irreparably damaged if there is an attempt to remove the applied label from the substrate. For instance, a label in which the holographic image layer is uncovered after application to a substrate and not protected by a top protective film layer would discourage attempts to remove the label from the substrate. Alternatively, an authentication label that does not have sufficient mechanical integrity to separate, lift, or otherwise detach, the label from a substrate without damage to the holographic image layer would discourage attempts to remove the label from the substrate. It would be desirable to produce an authentication label that becomes highly frangible as a result of the application of heat or by any other means of separation from the substrate, and thus prevent successful removal of the label with the holographic image undisturbed.